Night vision goggles enable a person to see objects in the darkness, which objects could not otherwise been seen by the naked eye. The principle by which night vision goggles operate is well known. Generally, some source of external light, such as the stars or moon, is capable of illuminating objects with very low light emitting therefrom. While the naked eye may not be able to detect such illuminations reflected from an object, the reflections can be electrically amplified by the night vision goggles so as to be visible to the observer's eye.
Therefore, the principle by which night vision goggles can make objects viewable in darkness is the electrical amplification of reflected light. An image of the object is displayed on a phosphorescent screen within the goggle. The battery powered photomultiplier or image intensifier tube is conventionally used to electrically amplify the light signals for presentation on the green phosphorous coated screen. The image is monochromatic, with the intensity of the phosphor representative of the amount of light reflected from the object. The phosphor coated screen is very sensitive and subject to accelerated degradation when exposed to high intensity light. Excessively lighted objects may thus overload the image tube and wash out the display on the phosphorous screen. Therefore, a need has arisen for a mechanism which automatically removes the battery power from the night vision goggle when subjected to periods of excessive light or to excessively lit objects.
In the prior art, this need has been addressed by circuits which remove battery voltage based solely on the intensity of the incident light. For example, in U.S. Pat. No. 4,755,725, there is disclosed one type of prior art light intensity monitor which senses the brightness of light to which the image intensifier tube is subjected.
As disclosed in the '725 patent, when the light intensity incident on the image intensifier tube reaches a preset threshold value, such as when the goggles are turned on in a lighted room, a timer is activated. If the light intensity exceeds the threshold value for more than one minute, for example, the timer is operative to open a switch to remove automatically power from the image intensifier tube, as well as from other circuitry.
The switch is driven by control logic circuits in response to the light intensity monitor. The switch includes a field effect transistor ("FET") placed in series with the battery and the image intensifier tube. To provide an extremely low series resistance, the switch is driven by a voltage which is larger than the battery voltage. The voltage multiplier is employed to boost the battery voltage to drive the FET switch.
An inherent disadvantage and limitation of prior art timer activated light intensity monitors, such as disclosed in the '725 patent, is that there is still a high potential of damage to the image intensifier tube during the timer countdown phase when the timing circuit is measuring the time during which the high light intensity condition exists. The high flux conditions which can be damaging to the image intensifier tube during the period in which the timer is counting down will not switch off the image intensifier tube in the prior art device. Furthermore, low flux conditions, although over the threshold intensity, may cause the image intensifier tube to be shut down after the time period has elapsed even though the total flux has not caused any damage.